Novel Trimmer Line for String Trimmers

ABSTRACT

A novel monofilament trimmer line is provided for reducing noise, reducing air drag and increasing durability by having a non-round cross-sectional shape defined as a polygon with three to six sides wherein the vertices between adjacent sides have been replaced with generous circular arcs, the arcs of a parabola, or even other arc shapes so long as the arc is convex and contains no sharp edges and wherein each arc is tangent to the two adjacent sides with a smooth and continuous transition, and the sides of the polygon are either straight or slightly convex, but not concave.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/772,166, filed Mar. 4, 2013 and U.S. Provisional Application No.61/906,465, filed Nov. 20, 2013, the entire disclosures of which arehereby incorporated herein by reference.

BACKGROUND

The invention is an improved string trimming line for rotary stringtrimmer devices. The trimmer line is attached to a disc (trimmer head)which is rotated by the trimming device. The trimmer line extendsessentially radially outward by virtue of the centrifugal forcegenerated by the rotation of the trimmer head attached to the lower endof the device. The primary use for the trimmer line is for cuttinggrass, weeds and vegetation. The rotational speed of the trimming lineis predominately determined by the motor attached to the machine but isalso influenced by the air drag on the line from spinning through theair.

String trimmers are known to operate at high rotational velocities of upto 10,000 revolutions per minute (rpm), although speeds from 5,000 to8,000 rpms are much more common. The high rotational speed of thetrimming line leads to considerable noise generation by the cuttingstring. To resolve this problem, a wide variety of proposals havealready been put forward. For example, U.S. Pat. No. 5,220,774 teaches ameans of designing a noise attenuating trimmer line wherein a spiralgroove or ridge is incorporated along the surface of an otherwise roundnon-twisted fiber.

Another solution to the noise problem, and the most common means toattenuate noise from a spinning trimmer line, is to manufacture atrimmer line with a twisted non-round cross-section. U.S. Pat. No.5,687,482, hereby incorporated by reference, granted to Behrendt isbelieved to be the oldest U.S. patent for a twisted monofilament linewith a non-round cross-section designed for the purpose of noiseattenuation in trimmer line. In addition to noise attenuation, Behrendtclaims that the concept also allows the trimmer machine to achievehigher rotational speeds, and thereby, a greater cutting performance.These types of line are commonly referred to as twist lines. Behrendtdepicts the twisting of a square cross-section with flat sides and sharpedges at the vertices of the sides (FIG. 1 a of the presentapplication). Similarly, U.S. Publication No. 2007/0256309 to Fogledepicts octagon (FIG. 21) and hexagon (FIG. 22) cross-sections for twisttrimmer lines having straight sides and sharp edges at the verticesbetween the sides (FIG. 1 d of the present application).

It should be noted that the known twist trimmer lines as well as thetwist trimmer lines of the present invention may be used with trimmingdevices powered by gasoline, propane, electricity or other commerciallyavailable sources of energy. In recent history, trimming devices havinggasoline motors have dominated the largest share of the U.S. market.Unfortunately, gas-powered motors generate so much noise that theimprovements achieved in attenuation of noise from the trimmer line havenot heretofore been a significant factor in the U.S. market. Even thoughthe majority of commercially available twist-shaped trimmer lines aretechnically effective at some level of noise attenuation, other factorssuch as durability and efficient use of power have become keydetermining factors for consumers.

Over the last few years, the use of lithium batteries for poweringstring trimming devices has grown significantly and continues to gainmarket share in the U.S. As compared to gas-powered trimming devices,battery-operated devices generate much less noise, and thus noiseattenuating properties of twist trimmer line may be more appreciated. Akey difference between gas-powered and battery-operated trimmingdevices, however, is that when a gas-powered device runs out of gas, auser can simply re-fill the tank and continue trimming vegetation.Because the batteries for trimming devices take one to three hours torecharge, a consumer notices when they have to stop trimming to rechargethe battery. Because of this, battery life is becoming a big factoramong consumers in determining which battery-powered trimmer topurchase. As such, the amount of drag reduction associated with atrimmer line is now a parameter which can be appreciated by the consumerbecause of its impact on battery life. A low-drag trimmer line canpositively impact the perceived value of the entire trimmer.

As noted above, when twist lines first became commercially available,one of the first noise-attenuating trimmer lines had a twisted-squarecross-section with sharp edges. Even though this twisted shapeattenuated noise, it had shortcomings. Most notably, the mass of thetwisted square shape was reduced by 35 to 40% compared to a roundfilament of the same outside diameter. This reduction in mass equated toless durability. A trimmer line's durability is a function of thecross-sectional shape, the production process, the materials comprisingthe line, and the mass of the line. To improve the durability of a twisttrimmer line, the twist trimmer line's mass must approach the same massas a round filament. In general, the space occupied by a cylindercircumscribing the twisted square shape should be the same or close tothe same size as the space occupied by the round filament.

Similar to the need to improve durability of noise-attenuating trimmerline is the need to decrease the air drag created by the line. Twisttrimmer lines have been made from a large variety of cross-sectionalshapes, and shaped monofilaments twisted along their longitudinal axisare known to attenuate noise relative to non-twisted lines of the samecross-sectional shape, when spun using string trimmers. However, outsideof the polygons of the prior art listed above, as well as oval-shapedtwist trimmer lines (U.S. Pat. No. 6,434,837), all of the proposedcross-sectional shapes for twist trimmer lines have either sharplongitudinal edges, concaved longitudinal grooves, and/or twistednon-round cross-sections with concaved longitudinal sides. Twisted lineswith sharp edges and/or grooves do attenuate noise when spun in the air,but they either do not reduce drag when spun in the air or they do notsufficiently reduce drag to the same degree as the noise is attenuated.

With the exception of oval lines, which by definition do not have welldefined edges, the prior art does not discuss the affect of applying agenerous radius to replace the sharp well defined edges that are nowcommonplace among commercial twist lines. More specifically, the priorat does not proposed adding a generous radius to twisted polygon shapes,nor does it define the benefit of doing so. Through experimentation ithas been found that the amount of radius applied where the sides of apolygon converge is an important factor in reducing drag and thusincreasing battery life of the device. Applying a generous radius wherethe sides of a polygon converge and then twisting the resultingcross-sectional shape can result in a line that both significantlyreduces drag as well as noise generated when spinning line in the air.The optimum amount of twist to reduce drag has been found to not be thesame as the amount of twist used to attenuate noise. For noiseattenuation, the preferred level of twist reported in the prior art istypically one revolution of twist per inch. For reducing drag, lesstwist per inch is preferred.

This invention identifies a new class of cross-sectional shapes fortrimmer line, which when twisted minimize the drag generated by thetrimmer line and yet also have more mass (more durability), whencompared to other twist line shapes of comparable size (defined by acircle/cylinder that circumscribes the twisted shapes). The presentinvention is focused on providing novel trimming lines of the type thatwill attenuate noise when spun at high speeds, maximize battery life dueto low drag, will spin in a flat plane, and yet be designed to eitherhave significant mass and/or made with materials that result in longlife.

BRIEF SUMMARY

This invention proposes a new class of twisted trimmer line withnon-round cross-sections—twisted polygons. The cross-sectional shapesare defined as polygons with three to six sides, where the verticesbetween adjacent sides have been replaced with generous circular arcs,the arcs of a parabola, or even other arc shapes so long as the arc isconvex and contains no sharp edges. Each arc is tangent to the twoadjacent sides with a smooth and continuous transition, and the sides ofthe polygon are essentially straight. The radius to apply to theconvergence point of two adjacent sides is defined as the ratio ofR1/D1, where R1 is the radius at the convergence point of two sides, andD1 is the diameter of a circle that circumscribes the entirecross-sectional shape. The ratio of R1/D1 should be at least around 0.05but could be as great as around 0.3. The preferred ratio of R1/D1 is0.2.

The polygon shapes of this invention are further defined in that thesides are not concaved. The sides should either be straight (flat) orbulge outward (convex). Additionally, the sides should not containconcaved longitudinal grooves. When these non-round shaped monofilamentsare twisted and spun longitudinally, the outermost portions of the shapedefine a cylinder. The amount of outwardly bulge of the sides should notexceed 50% of the distance (length) between the side of the polygon anda circle circumscribing the shape. When trimmer line is spun at highspeeds, sharp edges, longitudinal grooves and concaved sides result in aless aerodynamic profile and thus an increase in drag. The shape of theinvention's cross-section has been selected to make the trimmer linemore aerodynamic when spun at high velocities.

The filaments of this invention are also defined by the amount of twistor rotation of the cross-section shape about its central longitudinalaxis. The rotation is defined similarly to the definition used byBehrendt. The rotation can be described with the aid of an imaginaryline on the surface of the trimmer line (prior to rotation) which isparallel to the fiber's longitudinal axis and which touches a plane uponwhich the fiber is resting. One revolution is defined by two adjacentpoints on the imaginary line which are touching the same plane after thefiber is twisted. The invention is further defined as having onerevolution of twist between every 0.75 inches to one revolution of twistevery 3.0 inches. Ideally, the amount of twist would be one revolutionvery 1.7 inches.

One advantage of the invention is that the filaments cross-sectionalshapes and amount of twist have been selected to minimized air drag,maximize mass and thus optimize the life of a battery and the line'sdurability.

Another advantage of the invention is that the filament spins in a planewith minimal deviation from that plane and thus allows the machineoperator better control when edging and trimming. This performancefeature also means the line is less apt to tear the tips of fine grassand thus minimizes the browning of the grass.

Another advantage of the invention is that the material has beenselected to optimize durability of the filament allowing long life ofthe trimmer line independent of the other factors contributing to theline's durability.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the ensuing detailed descriptionand accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 a-d show twist trimmer line from the prior art comprising sharpedges between adjacent polygon sides;

FIG. 1 e shows a twist trimmer line from the prior art comprising agenerally triangular shape with rounded corners and three longitudinalgrooves;

FIGS. 2 a-f show a perspective view of six versions of a twist trimmerline of the present invention having a square-shaped cross-section;

FIGS. 3 a-f show a perspective view of six versions of a twist trimmerline of the present invention having a triangular-shaped cross-section;

FIGS. 4 a-f show a perspective view of six versions of a twist trimmerline of the present invention having a pentagonal-shaped cross-section;

FIGS. 5 a-f shows a perspective view of six versions of a twist trimmerline of the present invention having a hexagonal-shaped cross-section;

FIGS. 6 a-f shows an alternate embodiment of the twist trimmer lineshown in FIGS. 2 a-f;

FIGS. 7 a-f shows an alternate embodiment of the twist trimmer lineshown in FIGS. 3 a-f;

FIGS. 8 a-f shows an alternate embodiment of the twist trimmer lineshown in FIGS. 4 a-f;

FIGS. 9 a-f shows an alternate embodiment of the twist trimmer lineshown in FIGS. 5 a-f;

FIG. 10 is a side view of one of the versions of trimmer line shown inFIGS. 3 a-f;

FIG. 11 is a cross-sectional view of the trimmer line shown in FIG. 10;

Diagram 1 shows the cross-sectional shape of the twist line, thecharacteristics of which are shown in TABLES 1A and 1B;

Diagram 2 shows the cross-sectional shape of the twist line, thecharacteristics of which are shown in TABLES 2A and 2B;

Diagram 3 shows the cross-sectional shape of the twist line, thecharacteristics of which are shown in TABLES 3A and 3B; and

Diagram 4 shows the cross-sectional shape of the twist line, thecharacteristics of which are shown in TABLES 4A and 4B.

DETAILED DESCRIPTION

The cross-sectional shapes of the twist trimmer lines of this inventionconsist of polygons with three to six sides, with a generous radius atthe vertices between each of the two adjacent sides. The sides of thepolygons can range from generally flat (FIGS. 2-5) to outwardly bowed(convex) (FIGS. 6-9).

FIGS. 1 a-d are examples of prior twisted trimmer lines. The twistedfibers have different cross-sectional shapes, but there is no radius atthe vertices V_(a)-V_(d) between the adjacent sides of each polygon. Theedges form a sharp angle.

FIGS. 2 a-f depict a series of fibers with cross-sections that fallwithin the scope of the invention. The cross-sections of these fibersare generally square, but with a generous radius (R₁) replacing thevertices between the sides. If the diameter of a circle circumscribingthese cross-sections is labeled generally as D₁, then the radii (R₁) inFIGS. 2 a-f can be expressed by the ratio of R₁/D₁. For the twistedsquare-shaped filaments of this invention, this ratio (R₁/D₁) is definedas being less than or equal to 0.30 but greater than or equal to 0.05:

0.05≦R ₁ /D ₁≦0.30

In FIGS. 2 a-f, each radius at the vertices is defined by a portion of acircular arc A. These arcs are continuous and tangent to the twoadjacent sides; and in FIGS. 2 a-f, the sides S₁-S₄ are essentiallystraight or flat.

The value of diameter (D₁) circumscribing the fibers depicted in FIGS. 2a-f can range from 1.2 mm to 4.5 mm. However, for the examples in TABLE1A, the diameter D₁ is assigned the value of 2.4 mm. As a point ofreference, a solid round fiber with diameter 2.4 mm has an area (Area₀)of 4.52 mm².

TABLE 1A Theoretical Values for Twisted Square Cross-Sections withVarious Radii (R₁) FIG. 2f 2e 2d 2c 2b 2a 1a D₁ (mm) 2.4 2.4 2.4 2.4 2.42.4 2.4 R₁/D₁ 0.30 0.25 0.20 0.15 0.10 0.05 0 R₁ (mm) 0.72 0.6 0.5 0.360.24 0.12 0 Y₁ (mm) 0.14 0.18 0.21 0.25 0.28 0.32 0.35 Area₁ (mm)² 4.053.89 3.78 3.53 3.33 3.12 2.89 Area₁/Area₀ 0.90 0.86 0.83 0.78 0.74 0.690.64

In TABLE 1A, the length of the midpoint of each side radially outward tothe circle circumscribing the shape has been labeled as Y₁. The radius(R₁) in mm has been calculated by multiplying the diameter (D₁) by theratio R₁/D₁ shown in the table. The Area₁ was calculated based upon thegeometry and the other values shown in TABLE 1A. The final row of valuesin TABLE 1A is a ratio of the cross-sectional area of the invention(Area₁) divided by the cross-sectional area of a round fiber with thesame diameter as D₁ (Area₀).

TABLE 1A can be read as follows: The cross-section of the twisted fiberin FIG. 2 d can be circumscribed by a circle with diameter of 2.4 mm.The radius in each of the four corners is 0.5 mm. The distance from themid-point of each side radially outward to the circle circumscribing theshape is 0.21 mm. The cross-sectional area of this fiber is 3.78 mm²,which is 83% (Area₁/Area₀) of the area of a solid round fiber of thesame diameter.

It is important to note that the area of the trimmer lines (fibers) inTABLE 1A approach the area of a solid round circle as the radii (R₁) isincreased. More area equates to more mass which will improve the fiber'sdurability and useful life. However, these fibers retain their benefitsof reduced noise generation and reduced drag. Additionally, all of thefibers of the invention listed in TABLE 1A have larger cross-sectionalareas than the cross-section area of the fiber listed in FIG. 1 a whenthe fibers are sized to fit into the same circumscribing cylinder.

FIGS. 3 a-f depict a series of fibers with cross-sections that fallwithin the scope of this invention. The cross-sections of these fibersare all generally triangular, but with each vertex comprising a generouscircular arc defined by radius (R₁). If a circle circumscribing thesecross-sections is labeled generally as D₁, then the radii in FIGS. 3 a-fcan be expressed by the ratio of R₁/D₁. For the twistedtriangular-shaped filaments of this invention, this ratio (R₁/D₁) isdefined as being less than or equal to 0.30 but greater than or equal to0.05:

0.05≦R ₁ /D ₁≦0.30

In FIGS. 3 a-f, each radius at the vertices is defined by a portion of acircular arc. These arcs are continuous and tangent to the two adjacentsides; and in FIGS. 3 a-f, the sides are essentially straight or flat.

The value of diameter (D₁) circumscribing the fibers depicted in FIGS. 3a-f can range from 1.2 mm to 4.5 mm. However, for the examples in TABLE2A, the diameter D₁ is assigned the value of 2.4 mm. As a point ofreference, a solid round fiber with diameter 2.4 mm has an area (Area₀)of 4.52 mm².

TABLE 2A Theoretical Values for Twisted Triangular Cross-Sections withVarious Radii (R₁) FIG. 3f 3e 3d 3c 3b 3a 1b D₁ (mm) 2.4 2.4 2.4 2.4 2.42.4 2.4 R₁/D₁ 0.30 0.25 0.20 0.15 0.10 0.05 0 R₁ (mm) 0.72 0.6 0.5 0.360.24 0.12 0 Y₁ (mm) 0.24 0.3 0.35 0.42 0.48 0.54 0.6 Area₁ (mm)² 3.723.47 3.24 2.9 2.58 2.33 1.88 Area₁/Area₀ 0.82 0.77 0.72 0.64 0.57 0.520.42

In TABLE 2A, the length of the midpoint of each side radially outward tothe circle circumscribing the shape has been labeled as Y₁. The radius(R₁) in mm has been calculated by multiplying the diameter (D₁) by theratio R₁/D₁ shown in the table. The Area₁ was calculated based upon thegeometry and the other values shown in TABLE 2A. The final row of valuesin TABLE 2A is a ratio of the cross-sectional area of the invention(Area₁) divided by the cross-sectional area of a round fiber with thesame diameter as D₁ (Area₀).

TABLE 2A can be read as follows: The cross-section of the twisted fiberin FIG. 3 c can be circumscribed by a circle with diameter of 2.4 mm.The radius in each of the three corners is 0.36 mm. The distance fromthe mid-point of each side radially outward to the circle circumscribingthe shape is 0.42 mm. The cross-sectional area of this fiber is 2.9 mm²,which is 64% (Area₁/Area₀) of the area of a solid round fiber of thesame diameter.

FIGS. 4 a-f depict a series of fibers with cross-sections that fallwithin the scope of this invention. The cross-sections of these fibersare all generally pentagonal, but with each vertex comprising a generouscircular arc defined by radius (R₁). If a circle circumscribing thesecross-sections is labeled generally as D₁, then the radii in FIGS. 4 a-fcan be expressed by the ratio of R₁/D₁. For the twistedpentagonal-shaped filaments of this invention, this ratio (R₁/D₁) isdefined as being less than or equal to 0.30 but greater than or equal to0.05:

0.05≦R ₁ /D ₁≦0.30

In FIGS. 4 a-f, each radius at the vertices is defined by a portion of acircular arc. These arcs are continuous and tangent to the two adjacentsides; and in FIGS. 4 a-f, the sides are essentially straight or flat.

The value of diameter (D₁) circumscribing the fibers depicted in FIGS. 4a-f can range from 1.2 mm to 4.5 mm. However, for the examples in TABLE3A, the diameter D₁ is assigned the value of 2.4 mm. As a point ofreference, a solid round fiber with diameter 2.4 mm has an area (Area₀)of 4.52 mm².

TABLE 3A Theoretical Values for Twisted Pentagonal Cross-Sections withVarious Radii (R₁) FIG. 4f 4e 4d 4c 4b 4a 1c D₁ (mm) 2.4 2.4 2.4 2.4 2.42.4 2.4 R₁/D₁ 0.30 0.25 0.20 0.15 0.10 0.05 0 R₁ (mm) 0.72 0.6 0.5 0.360.24 0.12 0 Y₁ (mm) 0.09 0.11 0.13 0.16 0.18 0.21 0.23 Area₁ (mm)² 4.204.10 4.00 3.86 3.73 3.59 3.43 Area₁/Area₀ 0.93 0.91 0.88 0.85 0.83 0.790.76

In TABLE 3A, the length of the midpoint of each side radially outward tothe circle circumscribing the shape has been labeled as Y₁. The radius(R₁) in mm has been calculated by multiplying the diameter (D₁) by theratio R₁/D₁ shown in the table. The Area₁ was calculated based upon thegeometry and the other values shown in TABLE 3A. The final row of valuesin TABLE 3A is a ratio of the cross-sectional area of the invention(Area₁) divided by the cross-sectional area of a round fiber with thesame diameter as D₁ (Area₀).

TABLE 3A can be read as follows: The cross-section of the twisted fiberin FIG. 4 b can be circumscribed by a circle with diameter of 2.4 mm.The radius in each of the five corners is 0.24 mm. The distance from themid-point of each side radially outward to the circle circumscribing theshape is 0.18 mm. The cross-sectional area of this fiber is 3.73 mm²,which is 83% (Area₁/Area₀) of the area of a solid round fiber of thesame diameter.

Again, it is notable that as the radius R₁ is increased, the area of themonofilament fiber (invention) approaches the area of a round fiber(standard trimmer line) of the same circumscribing diameter. Whereas thearea of the twisted square invented by Behrendt had 64% of the area of acircumscribed circle of the same diameter, the example depicted by FIG.4 d has 88% of a circumscribe circle of the same diameter. This increasein area equates to an increase in mass, which equates to moredurability.

FIGS. 5 a-f depict a series of fibers with cross-sections that fallwithin the scope of this invention. The cross-sections of these fibersare all generally hexagonal, but with each vertex comprising a generouscircular arc defined by radius (R₁). If a circle circumscribing thesecross-sections is labeled generally as D₁, then the radii in FIGS. 5 a-fcan be expressed by the ratio of R₁/D₁. For the twisted hexagonal-shapedfilaments of this invention, this ratio (R₁/D₁) is defined as being lessthan or equal to 0.30 but greater than or equal to 0.05:

0.05≦R ₁ /D ₁≦0.30

In FIGS. 5 a-f, each radius at the vertices is defined by a portion of acircular arc. These arcs are continuous and tangent to the two adjacentsides; and in FIGS. 5 a-f, the sides are essentially straight or flat.

The value of diameter (D₁) circumscribing the fibers depicted in FIGS. 5a-f can range from 1.2 mm to 4.5 mm. However, for the examples in TABLE4A, the diameter D₁ is assigned the value of 2.4 mm. As a point ofreference, a solid round fiber with diameter 2.4 mm has an area (Area₀)of 4.52 mm².

TABLE 4A Theoretical Values for Twisted Hexagonal Cross-Sections withVarious Radii (R₁) FIG. 5f 5e 5d 5c 5b 5a 1d D₁ (mm) 2.4 2.4 2.4 2.4 2.42.4 2.4 R₁/D₁ 0.30 0.25 0.20 0.15 0.10 0.05 0 R₁ (mm) 0.72 0.6 0.5 0.360.24 0.12 0 Y₁ (mm) 0.06 0.08 0.09 0.11 0.13 0.14 0.16 Area₁ (mm)² 4.294.22 4.16 4.05 3.96 3.86 3.75 Area₁/Area₀ 0.95 0.93 0.92 0.90 0.88 0.850.83

In TABLE 4A, the length of the midpoint of each side radially outward tothe circle circumscribing the shape has been labeled as Y₁. The radius(R₁) in mm has been calculated by multiplying the diameter (D₁) by theratio R₁/D₁ shown in the table. The Area₁ was calculated based upon thegeometry and the other values shown in TABLE 4A. The final row of valuesin TABLE 4A is a ratio of the cross-sectional area of the invention(Area₁) divided by the cross-sectional area of a round fiber with thesame diameter as D₁ (Area₀).

TABLE 4A can be read as follows: The cross-section of the twisted fiberin FIG. 5 e can be circumscribed by a circle with diameter of 2.4 mm.The radius in each of the six corners is 0.6 mm. The distance from themid-point of each side radially outward to the circle circumscribing theshape is 0.08 mm. The cross-sectional area of this fiber is 4.22 mm²,which is 93% (Area₁/Area₀) of the area of a solid round fiber of thesame diameter.

A modified class of shaped twisted lines defined by this inventionincludes the same shapes in FIGS. 2 a-f; FIGS. 3 a-f; FIGS. 4 a-fl andFIGS. 5 a-f, but modified to allow the straight sides of the polygons tobow radially outward (FIGS. 6 a-f; FIGS. 7 a-f; FIGS. 8 a-f and FIGS. 9a-f). Defined mathematically, the midpoints of the straight sides of thepolygons are allowed to bow outward by up to 50% of the distance Y₁listed in TABLES 1A, 2A, 3A and 4A. The visual representations in FIGS.6 a-f; FIGS. 7 a-f; FIGS. 8 a-f and FIGS. 9 a-f show the midpoint of thestraight sides of the polygons bowed outward by 33% of the distance Y₁as listed in the prior TABLES 1A, 2A, 3A and 4A. Diagram 1 shows avisual representation of the cross-sectional shape of the twisted linescharacterized in TABLE 1A and TABLE 1B. Similarly, Diagram 2 shows avisual representation of the cross-sectional shape of the twisted linescharacterized in TABLES 2A and 2B; Diagram 3 shows a visualrepresentation of the cross-sectional shape of the twisted linescharacterized in TABLES 3A and 3B; and Diagram 4 shows a visualrepresentation of the cross-sectional shape of the twisted linescharacterized in TABLES 4A and 4B, indicating the reference measurementsR₁, D₁ and Y₁ for each. The values for ratio R₁/D₁, distance Y₁, Area₁,and ratio of Area₁/Area₀ in TABLES 1A, 2A, 3A, 4A, 1B, 2B, 3B, and 4Bhave been rounded to two decimal places.

For example, TABLE 1A provides the values of Y₁ as previously definedfor twisted square shapes with a range of radii. As the radii (R₁)increases, the value of Y₁ decreases and the area increases. The data inTABLE 1A is representative of twisted fibers per FIGS. 2 a-f, where theshape is circumscribed by a 2.4 mm circle. If the midpoints of the sidesof these twisted squares are allowed to bow outward by 33% of the valueY₁ as shown in Table 1A, then the respective fiber shapes in FIGS. 2 a-fwill be transformed into the shapes shown in FIGS. 6 a-f.

The practical commercial value of D₁ for the fibers depicted in FIGS. 6a-f can range from 1.2 mm to 4.5 mm. However, for the examples in TABLE1B, the diameter D₁ is assigned the value of 2.4 mm. As a point ofreference, a solid round fiber with diameter 2.4 mm has an area (Area₀)of 4.52 mm².

As can be seen from TABLE 1B, the value of Y₁ is now reduced by 33%compared to the values in TABLE 1A. As a result, the area of each fiberhas increased relative to the examples in TABLE 1A. More area equates tomore mass, which improves the durability and useful life of the twistedfiber.

TABLE 1B Theoretical Values for Twisted Square Cross-Sections withVarious Radii and Bowed Sides FIG. 6f 6e 6d 6c 6b 6a 1a D₁ (mm) 2.4 2.42.4 2.4 2.4 2.4 2.4 R₁/D₁ 0.30 0.25 0.20 0.15 0.10 0.05 0 R₁ (mm) 0.720.6 0.5 0.36 0.24 0.12 0 R₁ (mm) 0.08 0.10 0.11 0.15 0.18 0.22 0.35Area₁ (mm)² 4.17 4.09 4.05 3.86 3.70 3.52 2.89 Area₁/Area₀ 0.92 0.9050.895 0.85 0.81 0.78 0.64

Likewise, FIGS. 7 a-f show six versions of the invention which areidentical to the versions shown in FIGS. 3 a-f, except that the sides ofthe triangular polygons are shown bowed outward by 33% of the values ofY₁ listed in TABLE 2A. TABLE 2B provides corresponding values for thesenew fibers based upon cross-sections that are circumscribed by a 2.4 mmcircle.

TABLE 2B Theoretical Values for Twisted Triangular Cross-Sections withVarious Radii and Bowed Sides FIG. 7f 7e 7d 7c 7b 7a 1b D₁ (mm) 2.4 2.42.4 2.4 2.4 2.4 2.4 R₁/D₁ 0.30 0.25 0.20 0.15 0.10 0.05 0 R₁ (mm) 0.720.6 0.5 0.36 0.24 0.12 0 Y₁ (mm) 0.16 0.20 0.24 0.28 0.32 0.36 0.6 Area₁(mm)² 3.89 3.71 3.53 3.33 3.13 2.92 1.88 Area₁/Area₀ 0.86 0.825 0.780.74 0.69 0.64 0.42

FIGS. 8 a-f show six versions of the invention which are identical tothe versions shown in FIGS. 4 a-f, except that the sides of thepentagons are shown bowed outward by 33% of the values of Y₁ listed inTABLE 3A. TABLE 3B provides corresponding values for these new fibersbased upon cross-sections that are circumscribed by a 2.4 mm circle.

TABLE 3B Theoretical Values for Twisted Pentagonal Cross-Sections withVarious Radii and Bowed Sides FIG. 8f 8e 8d 8c 8b 8a 1c D₁ (mm) 2.4 2.42.4 2.4 2.4 2.4 2.4 R₁/D₁ 0.30 0.25 0.20 0.15 0.10 0.05 0 R₁ (mm) 0.720.6 0.5 0.36 0.24 0.12 0 Y₁ (mm) 0.04 0.05 0.06 0.08 0.09 0.11 0.23Area₁ (mm)² 4.32 4.26 4.22 4.14 4.07 4.00 3.43 Area₁/Area₀ 0.96 0.940.93 0.92 0.90 0.88 0.76

FIGS. 9 a-f show six versions of the invention which are identical tothe versions shown in FIGS. 5 a-f, except that the sides of the hexagonsare shown bowed outward by 33% of the values of Yt listed in TABLE 4A.TABLE 4B provides corresponding values for these new fibers based uponcross-sections that are circumscribed by a 2.4 mm circle.

TABLE 4B Theoretical Values for Twisted Hexagonal Cross-Sections withVarious Radii and Bowed Sides FIG. 9f 9e 9d 9c 9b 9a 1d D₁ (mm) 2.4 2.42.4 2.4 2.4 2.4 2.4 R₁/D₁ 0.30 0.25 0.20 0.15 0.10 0.05 0 R₁ (mm) 0.720.6 0.5 0.36 0.24 0.12 0 Y₁ (mm) 0.04 0.05 0.06 0.08 0.09 0.11 0.16Area₁ (mm)² 4.34 4.30 4.25 4.17 4.11 4.00 3.75 Area₁/Area₀ 0.96 0.950.94 0.92 0.91 0.88 0.83

In the above examples, the arcs joining the adjacent sides of thepolygons are circular arcs. The choice of arcs for the inventionincludes circular arcs, the arcs of a parabola, and other arc shapes solong as the arc is convex and contains no sharp edges. Each arc istangent to the two adjacent sides with a smooth transition. As shown inFIGS. 6-9 and Diagrams 1-4, the midpoint of each side as defined by aline connecting the two consecutive radii could bulge outwardly by asmuch as 50% of the distance from the mid-point of each flat sideradially outward to a circle circumscribing the cross-section of fiber.

For polygons with an even number of sides (squares, hexagons and thelike) the size of the arc for every other vertex could be different fromthe arc used for the in-between vertices. Likewise, for polygons with aneven number of sides, the shape of the arc for every other vertex couldbe different from the shape of the vertices used for the in-betweenvertices.

Another aspect of the invention is that the cross-sectional shape isrotated uniformly along the length of the longitudinal axis. In allembodiments of the invention the cross-section is maintained constantand uniform over the entire longitudinal axis within the limits ofmanufacturing capabilities. However, the rotation of the cross-sectionrelative to the longitudinal axis varies continuously by a specificangular amount per unit of length. In a square cross-section defined inthe prior art, this results in the rotating trimmer line with four edgesextending helically (FIG. 1 a). For the twisted shapes of the presentinvention, no sharp edges would be extended helically. Instead, largeand smooth radii would extend helically separated by smooth sides (FIGS.2 a-2 f, as well as FIGS. 3-9). The sides are not recessed relative tothe arc at each vertex. The advantages of the new trimmer lines of thisinvention are that flow resistance or air drag on the rotating line isreduced, noise generated by the line's rotation is attenuated, fortrimmer machines with torque motors the rotational speed of the tool isincreased, the plane of rotation of the line is more uniform, and themass of the novel trimmer line approaches the mass of solid fiber withthe same circumferential diameter, thereby improving the durability ofthe trimmer line over twisted lines of the prior art.

A degree of twisting in the order of magnitude of 20 twists per meter(20 t/m) means that the rotating trimming line is twisted by 360 degreestwenty times over the one meter length. The minimum level of twistingdesired for this invention is 13.1 t/m; and, the maximum level of twistdesired is 52.5 t/m. This is equivalent to one revolution of twist (360degrees of rotation) every 0.75 to 3.00 inches. The ideal level of twistis 24.6 t/m, or one revolution of twist every 1.6 to 1.7 inches. Therotation can be described with the aid of an imaginary line on thesurface of the trimmer line (prior to rotation) which is parallel to thefiber's longitudinal axis and which touches a plane upon which the fiberis resting. One revolution (REV) is defined by two adjacent points onthe imaginary line which are touching the same plane after the fiber istwisted (FIG. 10). The amount of twist along the fiber's length wouldideally be constant within the limitations of the twisting process, buta variable level of twist would also be acceptable.

There are three ways to fabricate twist into monofilament trimmer line.One means is to rotate or spin the capillary in the spinneret plate fromwhich the molten plastic is extruded. A second means is to rotate thespool both axially and longitudinally as the filament is wound onto thespool. These first two means can be accomplished during the extrusionprocess used to manufacture the monofilament trimmer line. The thirdmeans to impart twist to a filament is to perform the twisting after themonofilament is manufactured. To twist a short length of trimmer line,one end is first clamped and secured. Then the other end of the filamentis rotated while the length of the fiber is exposed to sufficient heat.The twisted formation is constrained and allowed to cool. Alternately,the fiber can be twisted continuously by rotating the payoff spools toimpart rotation into the fiber, while at the same time removing linefrom the spool, passing it through a nip to hold the twist, exposing theline continuously to heat to set the shape, cooling the line, passing itthrough a second nip, and winding the line onto a take-up spool.

A preferred embodiment is a twisted equilateral triangle (three-sidedpolygon) where each of the three vertices has been replaced with agenerous circular arc.

A second preferred embodiment is a twisted square (four-sided polygon)where each of the four vertices has been replaced with a generouscircular arc.

A third preferred embodiment is a twisted pentagon (five-side polygon)where each of the five vertices has been replaced with a generouscircular arc.

A fourth preferred embodiment is a twisted hexagon (six-sided polygon)where each of the six vertices has been replaced with a generouscircular are.

Additional preferred embodiments include the above embodiments where thesides are equally bowed outward.

Testing

To measure the impact of a fiber's shape on battery life, the followingsetup was used. A model CGT400 string trimmer manufactured by CoreOutdoor Power was mounted on a stand such that the trimmer head wassuspended above the floor of the lab. A Shakespeare™ brand Fury™ trimmerhead was mounted on the CGT400 trimmer. The Fury™ head is designed tohold two lengths of trimmer line using two sets of dual clamps. For allof the examples below, the line samples were cut to have a blunt tip andcut to a length of 6.5 inches. A mark was made on all samples 5 inchesfrom the blunt tip. When installing the line in the Fury™ head, thismark was centered in the eyelet of the head so that 5 inches of lineextended from each eyelet. The two opposing eyelets are three inchesapart. Thus the line swatch was 13 inches.

The CGT400 trimmer is powered by lithium batteries. The unit wasoperated at low speed for the following examples. At low speed, thetrimmer spins trimmer lines at a constant rate of 5,005 rpms. Thecontrols for the CGT400 vary the amount of electricity used from thebattery to maintain the 5,005 rpms. If a particular trimmer line shapehas lots of air drag, then the demand for electricity on the batteryincreases and the battery runs out of power in a shorter period of time.If a different line shape is more aerodynamic, then less power isrequired to spin the line and the battery will run for a longer periodof time before electricity in the lithium battery is consumed.

For all of the tests discussed below, the same lithium battery and thesame battery charger were used. The battery took a minimum of threehours to charge for each test. The trimmer machine throttle was taped tomaintain the trimmer operating continuously. Once the charged batterywas inserted into the trimmer it would begin spinning the line samplesand would run until the battery was depleted. The same battery would befully recharged for the next test.

The noise generated by each sample of trimmer line was measured using adigital sound level meter model 407750 from Extech. The sound wasmeasured 58 inches from the trimmer head, just above the throttle of thetrimmer. The samples selected were all approximately 95 mils (2.4 mm) indiameter, or the cross-section could be circumscribed by a circle 95mils (2.4 mm) in diameter.

Example 1

A sample of round copolymer trimmer line labeled as having a 95 mildiameter was selected for testing. The diameter was measured and foundto range from 94 to 96 mils. Ten pieces of line were cut to 6.5 inchesand marked at 5 inches from a blunt cut end. A pair of lines wasinserted into the Fury™ head such that 5 inches of line extended fromeach of the two opposing eyelets. The swath was 13 inches. The batterywas inserted into the trimmer and it was allowed to operated until thelithium battery was depleted. The run time and the noise generated wererecorded. This process was repeated until five pairs of lines weretested. The results were as follows:

Battery Run Sample # Time (minutes) Noise (dB) 1 53.8 84.0 2 54.5 83.5 356.5 83.3 4 56.0 83.8 5 55.7 84.0 Avg. 55.3 83.7 Range 2.7 0.7Round line was tested because this is the most common shape of trimmerline.

Example 2

A sample of trimmer line was made with the cross-section shown in FIG. 3d. The line was not twisted. Eight pieces of line were cut to 6.5 inchesand marked at 5 inches from a blunt cut end. A pair of lines wasinserted into the Fury™ head such that 5 inches of line extended fromeach of the two eyelets. The swath was 13 inches. The battery wasinserted into the trimmer and it was allowed to operate until thelithium battery was depleted. The run time and the noise generated wererecorded. This process was repeated until four pairs of lines weretested. The results were as follows:

Battery Run Sample # Time (minutes) Noise (dB) 1 48.9 88.0 2 63.9 79.5 355.9 83.8 4 56.5 83.8 Avg. 56.3 83.7 Range 15.0 8.5

The average battery run time and the average noise generated by thisline was not statistically different from the round line. However, therange increased significantly. The increase in variation is thought tobe attributable to the possible variations in orientation of the line'scross-sectional shape within the eyelet. Depending on the orientation ofthe line, the line's leading edge might be a flat shape, a circular arc,or somewhere between these two options.

Example 3

A sample of trimmer line was made with the cross-section shown in FIG. 3d. The trimmer line was manufactured to have one resolution of twistevery 1.6 inch. Ten pieces of line were cut to 6.5 inches and marked at5 inches from a blunt cut end. A pair of lines was inserted into theFury™ head such that 5 inches of line extended from each of the twoeyelets. The swath was 13 inches. The battery was inserted into thetrimmer and it was allowed to operated until the lithium battery wasdepleted. The run time and the noise generated were recorded. Thisprocess was repeated until five pairs of lines were tested. The resultswere as follows:

Battery Run Sample # Time (minutes) Noise (dB) 1 61.2 70.0 2 59.9 72.3 362.1 72.5 4 61.8 72.0 5 59.0 72.5 Avg. 60.8 71.0 Range 3.1 2.5

In summary, the battery run time increased from 55.3 minutes for roundline to 60.8 minutes for the twisted line made with the cross-sectionalshape in FIG. 3 d. This is an increase of 5.5 minutes, or 10%. The rangefor the battery run time for this example was minimal and comparable tothe range for round line (Example 1). Additionally, the level of noisedropped from an average of 83.7 dB for round line down to 71.9 dB forthe twisted shape. This drop of 11.8 dB is perceived by the human ear asmore than a 50% reduction of the noise generated by the rotating line.

The rotating line was observed utilizing a strobe light. The rotatingline was observed to be spinning in a single plane.

Example 4

An alternate sample of twist trimmer line was tested. The cross-sectionis depicted in FIG. 1 e. It is similar to the cross-section shown inFIG. 3 e, except that it also has three longitudinal concaved grooved.These grooves were located at the midpoint of each side. The line wastwisted one revolution over 1.4 inches. Six pieces of line were cut to6.5 inches and marked at 5 inches from a blunt cut end. A pair of lineswas inserted into the Fury™ head such that 5 inches of line extendedfrom each of the two eyelets. The swath was 13 inches. The battery wasinserted into the trimmer and it was allowed to operated until thelithium battery was depleted. The run time and the noise generated wererecorded. This process was repeated until three pairs of lines weretested. The results were as follows:

Battery Run Sample # Time (minutes) Noise (dB) 1 60.4 78 2 56.5 81 356.9 80 Avg. 57.9 79.7 Range 3.8 3

The average battery run time for this sample was approximately mid-waybetween the run time for round line (Example 1) and the sample inExample 3 above. The noise generated was above the mid-point for thelines in Examples 1 and 3 above; close to the noise generated for roundline. Example 4 exhibits the negative impact of longitudinal grooves onboth battery life and noise generation.

Example 5

A sample of non-twisted trimmer line was obtained with a cross-sectionper FIG. 1 of U.S. Pat. D358535, which is a triangle with slightlyconcaved sides and truncated tips. The truncated tips result in sixsharp longitudinal edges. Six pieces of line were cut to 6.5 inches andmarked at 5 inches from a blunt cut end. A pair of lines was insertedinto the Fury™ head such that 5 inches of line extended from each of thetwo eyelets. The swath was 13 inches. The battery was inserted into thetrimmer and it was allowed to operated until the lithium battery wasdepleted. The run time and the noise generated were recorded. Thisprocess was repeated until three pairs of lines were tested. The resultswere as follows:

Battery Run Sample # Time (minutes) Noise (dB) 1 43.3 82 2 40.1 86 341.6 86 Avg. 41.7 84.7 Range 3.2 4.0

Unfortunately, this shape greatly reduced the average battery run time.The average battery run time was reduced by almost 25% compared to roundline and was 31% less than Example 3. Additionally, the noise generatedwas slightly greater than the noise associated with round line.

The rotating line was observed with a strobe light. The line wasobserved fluttering between multiple planes of rotation.

Example 6

The same sample of trimmer line used in Example 5 was twisted to a levelof one revolution every 0.75 inches. Two pieces of line were cut to 6.5inches and marked at 5 inches from a blunt cut end. This pair of lineswas inserted into the Fury™ head such that 5 inches of line extendedfrom each of the two eyelets. The swath was 13 inches. The battery wasinserted into the trimmer and it was allowed to operated until thelithium battery was depleted. The run time and the noise generated wererecorded. The lines were then removed and reinserted and tested again.This process was repeated a third time. The results were as follows:

Battery Run Sample # Time (minutes) Noise (dB) 1 48.7 71 2 47.6 71 346.0 71 Avg. 47.4 71 Range 2.7 0

By twisting the shape in FIG. 1 of U.S. D358535, the battery run timewas improved from 41.7 to 47.4 minutes relative to the same shaped linenot twisted. Twisting this shape greatly attenuated the noise generatedby the non-twisted shape (71 dB vs. 84.7 dB). However, the battery runtime is still much less than then 55.3 minutes of run time for roundline. The best performing triangular shaped line of these examples isthe twisted shape from Example 3—the battery run time was 60.8 minutes.The cross sectional shape in Example 3 had flat sides with no concavedgrooves and generous circular arcs at the vertices.

The noise generated in Example 6 was at the lowest level of all thesamples tested, but only slightly better than the line in Example 3.Example 3 and Example 6 are evidence that twisting a non-roundcross-sectional shaped trimmer line to attenuate noise does notcorrelate to a similar reduction in drag. Example 6 attenuated the mostnoise, but consumed much more of the battery life compared to Example 3.

The average battery life and noise generated for all six of the aboveexamples in summarized in TABLE 5 below:

TABLE 5 Data Summary for Examples 1-6 Avg. Battery Avg. Sound ExampleRun Time (Min.) Level (dB) 1 55.3 83.7 2 56.3 83.8 3 60.8 71.9 4 57.979.7 5 41.7 84.7 6 47.4 71.0

The data is also plotted below in Graph A. The preferred trimmer lineproperties would be fibers with higher Battery Run Times and lower SoundLevels. For this data set, Example 3 has the best performance.

It is important to note that a 10 dB drop in sound levels is perceivedby humans as a 50% drop in the level of noise generated. The drop from79.7 dB for Example 4 to 71.9 dB for Example 3 is significant.

Other variations are within the spirit of the present invention. Thus,while the invention is susceptible to various modifications andalternative constructions, a certain illustrated embodiment thereof isshown in the drawings and has been described above in detail. It shouldbe understood, however, that there is no intention to limit theinvention to the specific form or forms disclosed, but on the contrary,the intention is to cover all modifications, alternative constructions,and equivalents falling within the spirit and scope of the invention, asdefined in the appended claims.

All references, including publications, patent applications, andpatents, cited herein are hereby incorporated by reference to the sameextent as if each reference were individually and specifically indicatedto be incorporated by reference and were set forth in its entiretyherein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) are to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. The term “connected” is to beconstrued as partly or wholly contained within, attached to, or joinedtogether, even if there is something intervening. Recitation of rangesof values herein are merely intended to serve as a shorthand method ofreferring individually to each separate value falling within the range,unless otherwise indicated herein, and each separate value isincorporated into the specification as if it were individually recitedherein. All methods described herein can be performed in any suitableorder unless otherwise indicated herein or otherwise clearlycontradicted by context. The use of any and all examples, or exemplarylanguage (e.g., “such as”) provided herein, is intended merely to betterilluminate embodiments of the invention and does not pose a limitationon the scope of the invention unless otherwise claimed. No language inthe specification should be construed as indicating any non-claimedelement as essential to the practice of the invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

What is claimed is:
 1. An elongated length of trimmer line for use witha powered rotary trimmer device comprising: A filament body having anon-circular cross-sectional shape comprising at least three distinctsides wherein each of said sides is essentially straight and having alongitudinal axis; Vertices equal in number to the number of distinctsides, said vertices located where adjacent sides converge andcomprising a convex arc contour Said filament body rotated about itslongitudinal axis.
 2. An elongated length of trimmer line as claimed inclaim 1, wherein said non-circular cross-sectional shape furthercomprises a polygon having three to six vertices.
 3. An elongated lengthof trimmer line as claimed in claim 1, wherein said convex arc contoursof said vertices comprise generous circular arcs, the arcs of aparabola, or other convex arcuate shapes and wherein each such shape istangent to the two adjacent sides with a smooth and continuoustransition.
 4. An elongated length of trimmer line as claimed in claim1, wherein the arc has a radius R, and the ratio of the arc radius (R)to the diameter of a circle capable of circumscribing thecross-sectional shape of the filament body (D) falls within the range of0.05 to 0.30.
 5. An elongated length of trimmer line as claimed in claim4, wherein the ratio of the arc radius (R) to the diameter of a circlecircumscribing the cross-sectional shape of the filament body (D) isabout 0.20.
 6. An elongated length of trimmer line as claimed in claim1, wherein said filament body is rotated about its longitudinal axis ata rate of not more than one full 360 degree revolution of the filamentbody per 0.75 inches of trimmer line.
 7. An elongated length of trimmerline as claimed in claim 1, wherein said filament body is rotated aboutits longitudinal axis at a rate of between about 0.75 inches to about3.00 inches of trimmer line per full 360 degree revolution of thefilament body.
 8. An elongated length of trimmer line as claimed inclaim 4, wherein said filament body is rotated about its longitudinalaxis at a rate of between about 0.75 inches to about 3.00 inches oftrimmer line per full 360 degree revolution of the filament body.
 9. Anelongated length of trimmer line as claimed in claim 7 or 8, whereinsaid filament body is rotated about its longitudinal axis at a rate ofbetween about 1.6 and 1.7 inches of trimmer line per full 360 degreerevolution of the filament body.
 10. An elongated length of trimmer lineas claimed in claim 1 or 8, wherein said cross-sectional shape sidescomprise a convex contour.
 11. An elongated length of trimmer line asclaimed in claim 10, wherein said convex contour does not exceed 50% ofthe distance between said cross-sectional shape side and a circlecircumscribing the cross-sectional shape of the filament body whenmeasured at a mid-point of the side.
 12. An elongated length of trimmerline as claimed in 11, wherein said convex contour is 33% of thedistance between said cross-sectional shape side and a circlecircumscribing the cross-sectional shape of the filament body whenmeasured at a mid-point of the side.
 13. An elongated length of trimmerline as claimed in claims 1, 8 or 11, wherein the sides of thecross-sectional shape are free of convex grooves.